One prior-art flow amplifier (cf. U.S. Pat. No. 4,356,759, Cl. B62D 5/08, 1982) is known to comprise a three-position hydraulically operated spool-type directional control device and a flow intensification system incorporating three variable flow restrictors or chokes. The directional control device is in fact a hydraulic directional control valve involving two sliding spool valves of which one is a reversing and the other, a regulating one. The reversing spool valve is a three-position one and effects alteration of the direction of flows in the hydraulic lines of the actuating hydraulic cylinder and in the internal hydraulic pilot lines of the directional control valve. The regulation spool valve provides for precise operation of the flow amplifier.
Flow intensification is attained due to an outside power source using the principle of a throttle or restrictive control, that is, maintaining the same differential pressure on both of the flow restrictors, of which the one having a smaller restriction area (the pilot restrictor) is supplied along the hydraulic pilot lines of the hydraulically operated steering wheel, while the other one having a larger restriction area (the intensifying restrictor), is supplied from an outside power source. The same differential pressure on both of the flow restrictors is maintained by changing the restriction area of a third (regulating) flow restrictor interposed between the external power source and the intensifying flow restrictor. The regulating flow restrictor is essentially a metering or throttling land of the regulating spool valve provided in the hydraulic pressure line of the outside power source before the inlet of the intensifying flow restrictor. Each of the end chambers of the regulating spool valve confined within its ends and the sleeve walls, communicates with the input of the respective pilot or intensifying flow restrictor. Then a total (amplified) flow from the pilot and intensifying restrictors is admitted to the reversing spool valve and further on to the actuating hydraulic cylinder of the system, whereupon the hydraulic fluid is expelled, from the opposite chamber of said cylinder, into the return line to be drained back into the tank.
However, the aforediscussed known flow amplifier suffers from too low operating precision on low-rate flows and high power losses on high-rate flows, a disadvantage accounted for by the fact that no automatic control of the restriction areas of the pilot and intensifying flow restrictors is provided in response to a change in the rate (or intensity) of flow; in addition, the flow amplifier in question suffers from inadequate operating reliability, too high specific metal content, as well as technological and constructional complicacy due to the provision of a two-spool valve directional control device.
Known in the present state of the art is also a flow amplifier in the hydraulic steering system of a transport vehicle (cf. a prospectus of Danfoss Co., Denmark, No. 6/82-02, 1982), featuring the same constructional arrangement as described hereinabove, i.e., it comprises a three-position hydraulically operated spool-type directional control device incorporating two sliding spool valves, a regulating and a valving (directional-control), and internal hydraulic pilot lines. The housing of the flow amplifier has a number of external portholes adapted for communicating the internal hydraulic pilot lines with the hydraulic power lines (i.e., pressure, return (exhaust), and cylinder supply lines) of the hydraulic steering system, and with its two external hydraulic pilot lines. The flow amplification circuit incorporates three variable flow restrictors or chokes, viz., a regulating, an intensifying, and a pilot one, which are provided in the regulating spool valve of the three-position hydraulically operated spool-type directional control device with a possibility of varying their restriction areas along the direction of the sliding spool valve travel.
The aforegiven constructional arrangement of the flow amplifier provides for an automatic control of the restriction areas in the pilot and intensifying flow restrictors in response to a change in the flow intensity. This is attained due to the fact that the flow restrictors are in fact the throttling lands of the regulating spool valve whose position and hence the opening of the pilot and intensifying flow restrictors, depends on the intensify of the hydraulic fluid flow. This makes it possible to maintain adequate operating precision of the flow amplifier on low-intensity flows and to minimize power losses when operating on high-intensity flows. However, inherent in the aforediscussed constructional arrangement of the flow amplifier are such disadvantages as too low operating reliability, high specific metal content, as well as technological and constructional complicacy due to the provision of a two-spool valve three-position hydraulically operated spool-type directional control device.